Unsaturated carboxylic acids such as methacrylic acid, and the esters of such acids such as methyl methacrylate, are widely used for the production of corresponding polymers, resins and the like. Typically, a saturated monocarboxylic acid, such as propionic acid (PA), can be catalytically reacted with formaldehyde (FA) to produce an alpha, beta-ethylenically unsaturated monocarboxylic acid, such as methacrylic acid (MA), and water as a co-product. The produced alpha, beta-ethylenically unsaturated monocarboxylic acid can be esterified to a polymerizable, alpha, beta-ethylenically unsaturated monocarboxylic acid ester, such as methyl methacrylate (MMA).
MMA is a monomer containing a carbon-carbon double bond (&gt;C.dbd.C&lt;) and a carbonyl group ##STR1## Polymers derived from MMA are sometimes also referred to as "acrylic" or "acrylic-type" polymers. The MMA-type polymers have desirable transparency, weatherability and physical strength properties. Typical end-uses for MMA-derived polymers include acrylic sheet that can be fabricated into signs, advertising displays, lighting fixtures, glazing materials, structural panels and the like, molding resins for automobile, bus, truck and other vehicular tail-light lenses, plumbing and electrical fixtures and the like, as well as constituents of a variety of surface coatings, adhesives, inks, floor polishes and the like.
Generally, the condensation reaction to synthesize an alpha, beta-ethylenically unsaturated aliphatic monocarboxylic acid, such as MA, takes place in the vapor or gaseous phase and in the presence of a catalyst which can be basic, acidic, or substantially neutral. In the absence of the catalyst, reactants typically require addition of heat energy to overcome an "energy of activation" of the reaction, which can be a barrier to formation of the desired products. Also, in the instance where the reactants are chemically converted to a variety of products, a catalyst may tend to increase the rate of formation of one product relative to one or more of the other products. Such a catalyst is said to possess increased selectivity qualities, often a consideration when choosing a catalyst for commercial production purposes.
Reaction temperature plays an important role in the activity of a catalyst, another important consideration. At a particular temperature, for example, a commercially-acceptable percentage of the reactants might be converted to a desired product, with only a relatively minor percentage of the reactants being converted to undesired by-products. Typically, an increase in the temperature of the reaction not only tends to increase the rate at which the reactants are converted to the desired product or products, but also tends to increase the rate at which undesired by-products are produced as well.
Catalysts commonly used for reacting PA with FA to produce MA are alkali metals supported on silica. Typical catalysts of this type are disclosed in U.S. Pat. No. 4,147,718 to Gaenzler et al., U.S. Pat. No. 3,933,888 to Schlaefer, U.S. Pat. No. 3,840,587 to Pearson, U.S. Pat. No. 3,247,248 (see also Canadian Pat. No. 721,773) to Sims et al., and U.S. Pat. No. 3,014,958 to Koch et al.
These prior-art catalysts, while effecting condensation of PA with FA to produce MA, unfortunately also generate undesirable by-products that have to be separated from the produced MA. Relatively low conversion and/or selectivity performance values, together with relatively low catalyst useful-life values, are additional drawbacks.
Generally, when PA and FA are reacted in the vapor phase, and in the presence of a catalyst, to produce desired product MA and co-product H.sub.2 O, a variety of undesirable by-products are simultaneously produced as well. The more common of these undesirables are hereinafter referred to as by-product A (2,5-dimethyl-2-cylopenten-1-one), by-product B (2,4,4-trimethyl-gamma-butyrolactone), and by-product 3-P (3-pentanone). The presence of these by-products is generally undesirable because current MA-esterification and MMA-polymerization technology requires separation of these by-products either from the MA before it is esterified to MMA, or before the produced MMA is polymerized. It is additionally desirable to remove by-product A from the MA prior to esterification as the presence of this by-product tends to interfere with the desired formation of MMA. In particular, the presence of by-product A in the MA tends to cause an undesirable polymerization of MA and attendant separation problems. Loss of product also may become significant.
Accordingly, it would be desirable to have a catalyst which provides improved PA conversion, which decreases undesirable by-product generation, and which enhances useful catalyst life. The catalyst, and the catalyst support, of the present invention meet the foregoing desires.
Not only has the catalyst of the present invention been observed to be more active than conventional catalysts (i.e. the present catalyst has been observed to enable the MA-synthesizing, gas-phase, condensation reaction of PA with FA to take place at a relatively lower temperature for a given conversion); but the catalyst has also been observed to exhibit increased selectivity toward production of MA as well. While reduction of reaction temperature tends to increase the useful life of the catalyst per se, a reduced operating temperature may reduce overall operating costs as an added benefit. The reduction in the amounts of the undesirable by-products, moreover, tends to reduce, and may even eliminate, the costs attendant to (1) the removal of the undesirable by-products from the MA prior to esterification, and (2) the purification of the MMA prior to polymerization.